In the field of the reverse osmosis membrane, thin membranes have been examined on an industrial level. For example, as an asymmetric membrane, the Lobe membrane made of cellulose acetate is disclosed in U.S. Pat. No. 3,133,132 and U.S. Pat. No. 3,133,137. The surface skin layer of this membrane has a semipermeability, however it does not have a high separation property for smaller molecules such as gas molecules. On the other hand, as a composite membrane, a polyamide thin membrane formed on the porous supporting membrane by the interfacial polymerization method is disclosed in U.S. Pat. No. 4,277,344. The surface skin layer of this type of membrane has also a semipermeability, however it does not have a separation property for smaller molecules such as gas molecules, either.
Recently reducing the thickness of the macromolecule materials such as for separating gas molecule has been investigated. This is conducted taking into consideration providing asymmetric membranes and composite membranes having a practical mechanical strength.
In the case where a macromolecule material having a high separation factor is formed as a membrane on a proper porous supporting film, it is preferable that the membrane should have no defects and the thickness of the membrane should be not more than 100 nm so as to increase a gas permeating speed practically. U.S. Pat. No. 4,929,405 discloses that the thickness of a homogeneous fluorine-containing aromatic polyimide membrane is controlled to be smaller than the required limitation of not more than 100 nm or not more than 400 angstrom(A) in a water facial expansion method. However, it is impossible to manufacture membranes having the above-mentioned thickness on an industrial level. Various methods have been also disclosed in, for example, Japanese Laid Open Patent No. Tokkai-Hei 4-40223 and U.S. Pat. No. 4,230,463, in which a polymer solution is coated onto the proper porous supporting film and then dried to form a thin film. When such an ultra thin film without having pin-holes is manufactured on an industrial level, the manufacturing process becomes complicated, the yield deteriorates and the cost is raised. Moreover, the ultra thin film has a void structure with large hole diameters and cannot be manufactured on an industrial level. The application of the interfacial polymerization method also has been investigated (See, for example, J. Appl. Polym. Sci., 44 (1992)1087-1093). However, a sufficient permeability and separation property cannot be obtained. In addition, a membrane of the skin layer and that of the porous supporting film are not made of the same materials, thus causing detaching from each other at the interface and also deteriorating the strength of the membrane.
An asymmetric membrane has been investigated in various ways. Tai-Chung et al. reported in J. Memb. Sci., 88 (1994)21-36 discloses a hollow fiber membrane having a no-defects skin layer of 200-300 nm level. But a pinhole-free skin layer having a thickness of not more than 100 nm that satisfies the required limitation cannot be manufactured on an industrial level. Moreover, the method for manufacturing the asymmetric membrane is disclosed in U.S. Pat. No. 4,705,540 in which the asymmetric membrane is formed by drying the surface of a membrane and then soaking it in water. But this method has not come into practice. In other words, this method remains as a laboratory level method for forming membrane. Thus, it is difficult to provide membranes stably on an industrial level by this method. U.S. Pat. No. 4,880,441 and J. Appl. Polym. Sci., 41 (1990)713-733 etc disclose an asymmetric membrane (hollow fiber membrane) having skin layers which gradually become dense, but these membranes are also insufficient in the thickness as well as the method for industrial use. Moreover, these asymmetric membrane have a relatively dense porous layer located under the skin layer. There arises a problem such that as the permeability of the skin layer increases, the permeation resistance of the porous layer increases. In order to obtain a pinhole-free asymmetric membrane, Japanese Laid Open Patent Nos. Tokkai-Hei 5-049882 and Tokkai-Hei 5-146651 disclose the post-treatment method and Japanese Laid Open Patent No. Tokkai-Hei 5-184887 disclose the pre-treatment method. Moreover, U.S. Pat. Nos. 4,902,422, 5,085,676 and 5,165,963 disclose the improvement in the manufacturing process.
However, the above-mentioned methods have problems as follows: the manufacturing process is increased and becomes complicated; the manufacturing cost is raised; and it is difficult to stably manufacture membranes on an industrial level. If the semipermeable membrane is required to be manufactured for the practical use and on an industrial level and separated sufficiently, the above-mentioned methods of the prior art cannot provide the semipermeable membrane with the satisfactory structure. More specifically, first, in the case of a composite membrane, thin film is formed in the porous supporting film so that the strength of the interface is not sufficient. Additionally, the holes on the porous supporting film are filled in order to obtain a no-defect membrane so that the thickness of the membrane is required to be greater. Secondly, in the case of an asymmetric membrane, the skin layer and the porous layer are formed together so that they are not detached from each other on the interface. In other words, in this case, the strength of the membrane is sufficient. However, it is impossible to manufacture a skin layer having a thickness of not less than 100 nm and no deficiency on an industrial level. The structure is characterized in that the hole diameters are made to be gradually larger from the skin layer part to the porous part. Consequently, near the skin layer and the porous part, when the quantity of permeation is great, the permeation resistance is also great. As a result, sufficient permeability and separation properties cannot be provided.